It is known that the electric response from a Geiger-Muller tube is not directly proportional to the dose flow I. This does not represent a problem when the display of the result of the measurements is achieved with an analog system such as a needle voltmeter, but it turns out to be a problem when the display is digital.
This invention is designed to achieve a dosimeter-radiation meter which makes it possible to measure a dose flow or a dose of ionizing radiation with a digital display, the device being such that it displays a linear response from the signal that is transmitted by the sensor which is comprised of a Geiger tube and knowledge of the parameters that are proper to the Geiger tube. Those goals are met with a dosimeter-radiation meter which includes, according to the invention, a circuit for matching and shaping impulses that are transmitted by the Geiger tube, pure binary number digital coding circuits for digitally coding the particular parameters K and .tau. of the Geiger tube (K representing efficiency and .tau. dead time), parallel input and series output registers having inputs are connected respectively to the digital coding circuits, a selecting circuit for connecting the matching circuit and the register outputs and also ensuring connection between a microprocessor system and a dose flow display unit.
The selecting circuit includes an interrogation connection which comes from the calculating microprocessor, a monostable circuit connected to the interrogation connection and a switch to bring the interrogation connection in communication either with the matching circuit output to receive impulses that are transmitted by the formatted Geiger tube, or with the register output to receive coded data relating to the K and .tau. parameters.
Advantageously, the dosimeter includes a probe casing and a processing casing connected together. The probe casing includes the radiation sensor, the matching circuit, the coded K and .tau. parameter introduction circuit and the selecting circuit. The processing casing includes the microprocessor unit and the display unit.
According to a specific embodiment, the matching and shaping circuit includes a first low threshold comparator which ensures matching and saturation sensing, and second comparators in parallel. The inverting inputs of the comparators are connected to the output of the first comparator in order to format and ensure magnification of the signal that comes from the first comparator.
According to a specific implementation mode, the impulses that appear on the resisting load of the Geiger tube are applied to the inverting input of the first comparator. An initial high value resistor and the cathode of a diode are connected to the inverting input of the first comparator, while the second end of the resistor and the anode of the diode are connected to the non inverting input of the first comparator. A condensor is connected between the non inverting input of the first comparator and the ground, and poling resistors ensure application to the inverting and non inverting inputs of the comparator of positive poling voltage that is greater than the comparator threshold voltage.
According to another embodiment, the dosimeter includes means to maintain at saturation level the matching and shaping circuit when the Geiger tube operates under very intense exposure.
The invention also generally pertains to a method of linearizing the electric response from a radiation sensor like a Geiger-Muller tube in relation to the dose flow of ionizing radiation which crosses the tube, characterized in that a pure binary number digital code is established for the specific parameters K and .tau. of the Geiger tube K representing efficiency and .tau. dead time. First parameters K and .tau. are digitally stored into a memory. The number Ne of impulses transmitted by the Geiger tube are then counted for a preset measuring period. Finally, an altered number N.sub.S of impulses is calculated from the formula EQU N.sub.S =Ne/[K(1-.tau.Ne)] (1)
where a is a constant, and displaying an exposure dose flow from the calculated number N.sub.S of impulses.
According to this method, we can also conduct a deflection correction in the tracing of a dose flow curve in relation to time. According to the method, an altered N'.sub.S number of impulses is calculated from the formulas EQU N'.sub.S =1,207N.sub.S -60 (2)
if the number of altered N.sub.S impulses is in an average exposure zone and EQU N'.sub.S =0.707N.sub.S +1873 (3)
if the number N.sub.S of impulses is in a high exposure zone. An exposure dose flow can be displayed from the calculated number N'.sub.S of impulses.
Furthermore, we realized that the electric response from a Geiger-Muller tube displays equilibrium deficiencies when the temperature changes, especially below 0.degree. C.
Another feature of the dosimeter according to the invention rests in the fact that it includes a thermostat structure inside which the radiation sensor is placed, together with a heating resistor coiled around it made of an alloy that contains a high copper content coiled around it, and a thermistor for temperature sensing. Preferably, the coiled heating resistor is surrounded by two metal half-shells.
Other features and advantages will be realized in the description that follows of a particular embodiment of the invention, with reference to the attached drawings.